In recent years, a control system connecting various control object devices with each other through a network has been employed, in order to attain wiring saving. Particularly in an industrial machine, a distributed control system connecting drivers driving motors of respective shafts constituting a driving system thereof and input/output devices included in peripheral units with each other through a network and controlling these control object devices is employed.
It is known that such a distributed control system is constituted of a central communication apparatus unifying the whole sequence and a plurality of terminal communication apparatuses executing input/output control to/from control object devices such as sensors and actuators. The central communication apparatus and the respective terminal communication apparatuses are connected with each other through communication lines in a mode such as a multidrop mode or a daisy-chain mode, whereby a network is constituted.
In the distributed control system, the central communication apparatus and the terminal communication apparatuses transmit/receive control information for the control object devices and sensor input information to/from each other through such a network, thereby executing control of the control object devices.
Ring-type and bus-type connection modes according to the multidrop mode are frequently employed for the network of such a distributed control system, and these connection modes have such advantages that communication control is easy and construction of the network is also easy.
At present, however, the number of control object devices required for an industrial machine increases following high functionalization and high performance of the industrial machine, and large scaling progresses also for the system.
In application of the distributed control system to such a large-scale system, restriction of the connection mode is remarkable in a network of the multidrop mode or the like, and the structure of the system is extremely complicated.
In order to solve such a problem, a technique of simplifying multipoint connection by constituting a tree-type network according to point-to-point connection for improving the degree of freedom in the connection mode of the network is conceivable. In such a network, however, a communication delay time varies with the connection mode (hierarchy) of the network, and hence synchronous control of inputs/outputs in a plurality of control object devices becomes difficult.
In a conventional distributed control system, it is necessary to uniformly perform communication in response to exhibiting highest performance and also in a case where different control cycles are required to respective ones of a plurality of synchronous control operations on the same network. An excessive communication speed is required for the network in order to correctly guarantee these different control cycles in the conventional distributed control system, and hence execution of strict synchronous control is difficult.
With respect to such a problem, a technique described in Japanese Patent Laying-Open No. 2009-60480 is disclosed, for example. A field control system described in Japanese Patent Laying-Open No. 2009-60480 senses delays in control processing, and collects measurement result packets to which time stamps of respective field devices are added. Then, this field control system grasps communication delay times between the respective field devices on the basis of the time stamps. Further, the field control system adjusts operation schedules of the respective field devices in response to these communication delay times.